Field of the Invention
The present invention is directed generally to multi-layer combined rigid and flexible printed circuit configurations in which flexible sections extend from or join one or more rigid sections normally as part of the interconnect system used in electronic packaging.
Description of the Related Art
Processes and techniques for manufacturing multi-layer printed circuit devices have been evolving over a long period of time. In this regard, flexible circuits have been widely recognized as a substitute for wire harnesses because they are lighter, have greater flexibility, greater reliability and require less labor to assemble. As the complexity and the electrical performance requirements of printed circuits increased over a period of time there evolved a reduction in the trace widths, trace spaces and pad-to-hole ratio and, simultaneously, an increase in the complexity or number of conductor layers utilized. To meet this challenge, multi-layer flexible and rigid-flex circuits were developed.
Conventional flexible printed circuit material for rigid-flex manufacturing include Kapton (a trademark of E. I. Du Pont De Nemours and Company of wilmington, Delaware) for polyimide dielectric flexible film and acrylic adhesive materials. Other dielectric flexible files used for fabrication flexible printed circuits include Nomex, Mylar, for a polyester material and Teflon, for polytetrafluoroethylene, (all are also trademarks of Du Pont). Conventional flexible laminates consist of a flexible dielectric film such as polyimide film bonded to a copper substrate using a flexible type of adhesive. The conductive patterns on the flexible materials are formed by a print and etch operation. The etched flex layers are then laminated using heat and pressure with a cover layer material which includes a flexible dielectric film attached with a flexible type of adhesive.
Rigid printed circuit laminates are normally copper clad materials using an epoxy or a polyimide laminate. The conductive patterns on the rigid materials are also formed by a print and etch operation. The etched rigid layers are then laminated with other layers to make a rigid-flex printed circuit.
Multi-layer flexible circuits are normally built by using multiple individual flexible layers sandwiched using a flexible adhesive material for flexibility or with fiberglass sheets impregnated with a resin or adhesive such as epoxy and are known as "prepreg" (also referred to as a B-stage).
The rigid-flex printed circuits normally include individual flexible layers and rigid layers stacked to form a many-layered composite. The flexible layers are an integral part of both the flexible and rigid portions of the device; whereas, the rigid layers are normally a part of only the rigid portion of the device. Adhesive normally used to bond the rigid and flexible layers in a rigid-flex circuit board is normally either a flexible type such as an acrylic/epoxy adhesive or a rigid type, for example, glass reinforced prepreg.
The flexible dielectric and adhesive materials such as those recited above exhibit excellent flexibility, stability and heat resistance properties and can be readily bonded to copper sheets for circuit pattern delineation. However, difficulties have been encountered because of the relatively high co-efficient of thermal expansion associated with these materials compared to copper, tin/lead conductor and connecting materials. Thus, when the printed circuits using these materials are subjected to cyclical thermal environment, the flexible type of material expands and contracts at a much higher rate than the other materials used in the printed circuit fabrication.
Normally, the various layers of patterned circuits in the rigid and the flexible laminate are connected together electrically by utilizing "through-holes" which are holes drilled in the board and extending down through all the layers which are subsequently plated with a conducting material to form a coating on the inside of the hole in the form of a conductive layer which forms a common connection joining the layers connected by the given through-hole. The conductor is normally a copper or solder material depending on the application.
Because the flexible materials in the composite rigid-flex board expand at a much higher rate than the rigid materials, the expansion during soldering or other heat-related operations which take place after plating or cladding of the through-holes tends to create a great deal of stress relative to the connections between the various levels of patterned circuitry in the through-holes. If expansion along the through-holes in the direction normal to the plane of the laminated sandwich is sufficient, the integrity of the plated conductor in the holes may be broken. This also occurs after repeated cyclical stresses over a specified temperature range. In any event, this phenomenon causes failures in the printed circuits including a much higher rate of manufacturing rejection than is desirable.
One approach to solving this problem is illustrated and described in U.S. Pat. No. 4,687,695 which describes a process for making flexible printed circuits which addresses the problem of fabricating through-holes in the rigid areas of the flexible circuit. That solution involves substituting epoxy glass or conventional rigid printed circuit materials for the flexible printed circuit material in the rigid areas of the flexible printed circuits. This process avoids the process of plating through-holes in flexible printed circuit materials at all.